Nonwoven substrates (also called simply “nonwovens”) are webs and or three-dimensional structures of individual fibers or threads, which are interlaid but not in an identifiable manner typical of a knitted or woven substrate. Historically, nonwovens have been made by one of several different processes, such as melt blowing, spunbonding, air-laying, wet-laying, co-forming, carding, needle punching, spunlacing, and the like. The base weight of nonwovens is expressed in ounces per square yard (osy) or grams per square meter (gsm), and the fiber diameters may be measured in deniers, microns, or nanometers (ranging in size from highest to lowest). The term “mat” may be used interchangeably with the term “web” to refer to a nonwoven substrate.
Melt blowing is a major manufacturing technology for producing fibers for nonwoven webs. The term “melt blown” refers to fibers or a mat formed by extruding a molten thermoplastic material (the “melt” or the “polymer melt”) through a plurality of fine orifices as molten filaments into converging flows of high-speed heated gas in a process described more fully herein.
Melt blown webs are produced by extrusion of a polymer melt, as mentioned above. The term “polymer” includes homopolymers, copolymers (such as, for example, block, graft, random and alternating copolymers), terpolymers, and blends and modifications thereof. Furthermore, the term “polymer” shall include all possible geometric configurations of the molecule. These configurations include molecules with isotactic and random symmetries.
Nonwoven webs produced by melt blowing are used in a wide variety of applications, ranging from aerospace to medical to industrial uses, where an integral protective substrate is desired or required. One exemplary product made by nonwovens is filter media.
The manufacture of nonwoven webs by melt blowing has been described in U.S. Pat. No. 3,825,380; U.S. Pat. No. 3,849,241; and U.S. Pat. No. 4,889,476, for example, as well as in numerous other publications. Generally speaking and without reference to any specific patent or other publication, the polymer is melted in an extruder and forced through a row of fine capillaries (known as “orifices” or “nozzles”) to produce molten filaments. The orifices are defined through the apex of a sharp angled metal structure called the “die tip.” The die tip is surrounded by two adjacent parts known as “air plates” or “air knife plates” that define a gap between the plates and the die tip that constitutes the geometry of the “air knives.” Together, the die tip, the orifices, and the air plates are referred to as the “die tip assembly.”
High pressure, high temperature air (the “primary air”) passes through the gap between the air plates and the die tip to form the air knives. The pressure of the supplied primary air determines the blowing speed of the air knives. The air knives attenuate the molten filaments as they exit the orifices to reduce their diameters and to improve the molecular alignment of the polymer. By regulating the temperature and pressure of the primary air and polymer melt, this arrangement is capable of producing fibers of different diameter sizes, from the large micron to the sub-micron diameter range.
Specifically, the melt blowing process can be used to make fibers of various diameters, including macro fibers (average diameters from 40 to 100 microns), textile-type fibers (10 to 40 microns), microfibers (1 to 10 microns), ultra-fine microfibers (under 3 microns), submicron fibers (less than 1 micron) and nano-fibers (0.2 to 0.7 microns). Melt blown fibers may be continuous or discontinuous, and are often self-bonding when deposited onto a collecting surface.
After the molten polymer passes through the die tip orifices and is contacted by the air knives, the air-borne molten filaments begin to solidify and are cooled by unheated or cooled ambient air (referred to herein as the “cooling air” or the “secondary air”) in a step known as “quenching.” In addition to cooling the polymer, quenching also impacts the molecular properties of the fibers, such as crystallization. The fibers are then randomly deposited onto a collector's moving porous surface, such as a rotating drum or a conveyor belt—often assisted with suction beneath the drum or belt surface—to form a nonwoven web, mat or article with significant integrity resulting from a high degree of fiber entanglement and bond.
For making products of considerable width, a device called a “die” (or “die body”) can be used to spread the hot melt flow coming from the extruder into a uniform slit flow of desired width, which is suitable to be fed into the die tip. The most advanced die type is the “coat hanger die,” as described in U.S. Pat. No. 4,285,655. Other types of die bodies currently in use include a linearly tapered die, a slot die, a T-shaped die, a fishtail die, and the like.
Because of its ability to produce micro-fibers useful in a large variety of products, melt blowing technology enjoys continued popularity in industry. However, many significant shortcomings remain, many of which are documented below:
A. Melt blowing is energy intensive. Large amounts of energy are required for pressurizing, heating, and distributing the primary air streams. Mainly as a result of the design of existing melt blowing equipment, the primary air must be heated to temperatures near that of the polymer melt flow. The temperatures of the polymer melt flow in the die and the primary air must have a minimal differential in order to maintain the thermal equilibrium and homogeneous viscosity of the polymer melt. Producing a large volume of high temperature air (that is, the primary air flow) is expensive and leads to troublesome manufacturing consequences.
One consequence is that the secondary air used to quench the newly extruded fibers must be sufficiently cool and of sufficiently large volume to offset the heat of the primary air in order to effectively quench and solidify the fibers. If the secondary air is not sufficiently cool or of a sufficiently large volume, the resulting web has a harsh texture with embedded shots. “Shots” are small beads of re-melted fibers and/or polymer melt that failed to be attenuated into discrete fibers. In addition to affecting the texture of the web, shots cause the web to have poor appearance, opacity, coverage, strength, and filtration efficiency.
Another problem is that slow solidification and over-attenuation by the hot primary air may turn some fibers into “flies.” Flies are tiny broken fibrous bits and pieces that escape the main fiber stream. Flies contaminate the product and the equipment, as well as causing a hazardous work environment.
Also, the consumed primary air supply undesirably heats up the manufacturing environment. As a result, it is often necessary to prepare and supply additional secondary cooling air. Ventilation, air conditioning, and air balancing means may be required during hot hours and hot days. Such additional cooling means can be costly and cumbersome.
U.S. Pat. No. 4,112,159 acknowledges some of the benefits of having cooler primary air but did not disclose a way to achieve it. U.S. Pat. No. 4,526,733 suggests using insulation between the die tip apex and the adjacent airflow for the said benefits. U.S. Pat. No. 6,336,801 suggests special heater devices and insulation to be installed on or inside the die tip apex to allow for cooler primary air flows. None of these methods has been widely accepted, because the die tip apex is only a small part of the many surface areas where thermal interference may occur and because the methods proposed are complicated and costly. Additional operation and maintenance incurred may be burdensome. More effective and simpler means to thermally separate the primary air flow and the melt flow are still needed.
B. Each polymer type has its own unique quench requirement for molecular crystallization soon after molten filaments are formed through the orifices. Meeting this requirement is important to fiber formation and its final quality. With some polymers, the necessary temperature differential between the molten filaments and the quenching air is so large that most conventional melt blowing equipment cannot maintain the requisite temperature differential. As a result, many polymer types are unsuitable for use in the currently available melt blowing equipment.
By way of example, newly developed bio-based polymers (made from organic materials or biomass) are of particular interest in medical applications, where the fiber polymer may be compounded with medicine. The resulting nonwoven web may be applied to or below the skin of a patient, such that the fibers and medicine may be dissolved and absorbed by the patient's body at a desired and controlled rate. This delivery mechanism results in a valuable medical device.
To date, the difficulty in producing this type of medical device is that the bio-based polymers have a low crystallization rate that requires a relatively large temperature differential for quenching. Since the present melt blowing equipment is unable to allow for the needed temperature differential between the primary air flow and the polymer melt flow, bio-based nonwoven substrates are expensive to produce and are available only in limited supply.
C. Conventional melt blowing equipment has low production capabilities with rigid ceilings because there are intrinsic limitations in both their melt and blowing systems. Specifically, the fine orifices at the apex of a die tip that issue molten filaments have limited structural strength to withstand melt pressure. Excessive pressure will crack the metal and allow the melt to ooze out in an uncontrolled manner. Die tip cracking is a messy and costly accident known by people in the industry as an “un-zipper.” There have been many efforts to reduce this risk. For example, U.S. Pat. No. 3,825,370 teaches the use of hypodermic tubings in place of drilled orifices; U.S. Pat. No. 4,486,161 teaches the use of tie bars to reinforce the die tip; and U.S. Pat. No. 4,986,743 teaches the use of a pre-stress method to do the same. Only moderate improvement was achieved.
The conventional “blowing” system also cannot tolerate additional primary air flow because it employs the so-called “manifold” technique repeatedly (as shown in FIG. 1) to convert the primary air into air knives of usable uniformity. This technique, also known as the “reservoir” or “dissipation” method, uses sudden expansion and restriction of the flow's cross-sectional area and/or abrupt turns to reduce the momentum of the air flow in exchange for flow uniformity in the width direction (CD). The restriction and/or turning of the air flow results in a large loss of kinetic energy. When airflow exceeds the designed volume, energy consumption rises steeply, and flow uniformity deteriorates.
Good air uniformity is harder to achieve on a wide apparatus because large manifolds and pipes naturally have large Reynolds numbers, which means erratic secondary flows will co-exist with the main flow. Also, the increased width of the apparatus makes the machine bulky and clumsy in all three dimensions. U.S. Pat. No. 5,080,569 provides an effort to hold down the size for wide machines. It employs a specially configured pressure control diverter built inside the manifold and additional control dampers installed outside downstream of the manifold to manipulate the uniformity of the flow. However, in industrial practices, the benefit gained is offset by the added complexity in equipment and operation.
D. In some instances, it is desirable to produce a composite nonwoven from multiple web forming processes simultaneously. For example, it is common for spun-bonded and melt blown nonwovens to be generated in tandem to form a composite nonwoven substrate having one or more layers of each type of nonwoven. Due to the limited production capability of a conventional melt blowing apparatus, multiple units used in tandem may be used in situations only when their combined production rate can keep pace with that of the co-working process. Such examples include SMMS and SMMMS processes, where “S” stands for a spunbonding apparatus and “M” for a melt blowing apparatus. But using too many melt blowing units in tandem is costly and cumbersome. Therefore, melt blowing units capable of higher production are desirable.
In addition to higher production capability, the multi-process users also want melt blowing apparatuses to have a smaller dimension in the machine direction, preferably less than 1 meter. The present norm is 1.5 to 5 meters, depending on machine's width. The reduced machine direction dimensions occupy less factory space or permit a more generous secondary air flow system (quench air) that is beneficial to process and product quality. U.S. Pat. No. 6,972,104 presents a design for a melt blowing apparatus having reduced machine direction dimension, but requires a complicated and costly internal structure.
E. The fine orifices on the die tip get contaminated gradually by impurities and gel in the melt flow during use. This die tip contamination leads to a progressive decline of process efficiency and product quality. Scheduled and unscheduled replacements of the die tip assembly must be performed. Replacing the die tip requires first shutting down and cooling-off the entire melt blowing apparatus, followed by disassembling, replacing, reassembling, and adjusting the die tip; reheating the die body and the primary air flow; restarting and readjusting the melt blowing apparatus; and finally zeroing in on the product specifications. These many steps waste a significant amount of labor, material, and time.
A method for improvement is described in U.S. Pat. No. 5,580,581 and U.S. Pat. No. 5,632,938, in which a special die tip assembly is designed to avoid disassembly and re-assembly works on the main die body. This die tip assembly may be installed onto, and replaced from, the die body without the aforementioned cooling off and reheating steps. However, this design involves extra parts and weight, cannot allow on-line adjustment of air gap, and does not address other issues discussed in this section.
F. Air gap setting and its evenness impact product quality directly. Therefore, it is desirable to periodically check the air gap setting and to make on-line corrections, if necessary, to reduce product defects and waste. U.S. Pat. No. 4,889,476; U.S. Pat. No. 5,080,569; and U.S. Pat. No. 5,248,247 allow for such gap setting and adjustment. Unfortunately, each of the prior art approaches requires shutting down and cooling off the entire machine to replace the die tip assembly. Thus, a design that can provide both benefits (E and F) is desired.
G. Due to the common presence of fibrous “flies,” the melt blowing apparatus and its surroundings need diligent and regular clean-up to avoid problems with safety, health, and product quality. To minimize flies and to facilitate cleaning, it is desirable to have an apparatus with a simpler configuration and a smaller size.
H. It is slow and expensive to build a new melt blowing apparatus. To rebuild an existing primary air system for any reason is also difficult and costly, because the air flow pathway built inside the die body and the die tip assembly is tortuous.
A more cost-effective and efficient design is desirable. Also wanted is a design that can properly process a wider range of polymer types. These improvements are likely to increase the nonwoven manufacturers' profit.
It is therefore an objective of the present invention to provide improved equipment design for the production of melt blown nonwovens. Specific goals include energy savings, simpler equipment, easier operation and maintenance, better product uniformity and quality, broader raw material choices, and smaller apparatus size.